Almanac β€” Help & Guide

What each tool does, and a short history of why the date of Easter is so hard to pin down.

The tools

πŸŒ™ Moon Phases
Every phase of the Moon for a year, with the dates and Universal Time of each new, first-quarter, full, and last-quarter moon. Lists full and new moons, flags blue moons (both the calendar-month and the seasonal definitions), and shows how the ecclesiastical Paschal full moon behind Easter compares with the real astronomical one. Works back to AD 30; pre-1582 dates are shown on the Julian calendar.
πŸ“… Date of Easter
Easter and the movable feasts for a range of years. Compare mode places the Western (Gregorian) and Eastern (Orthodox) dates side by side and marks the years they coincide; the single-tradition modes give the full feast table from Septuagesima to Holy Trinity.
✠ Movable Feasts
For a single year, every feast that moves with Easter β€” Septuagesima, Ash Wednesday, Lent, Holy Week, Ascension, Pentecost, Trinity, Corpus Christi β€” in both traditions.
πŸŒ‘ Passover & the Paschal Moon
The spring full moon (14 Nisan) reconstructed astronomically from the Exodus era to the present, on both the Gregorian and Julian calendars. This is a reconstruction of the sky, not a record of the historical calendar (see the accuracy note below).
πŸ“Š Easter Statistics
Over any range of years: the earliest and latest Easter, the full distribution of dates, and how often Western and Eastern Easter fall together. Western Easter can never be earlier than 22 March or later than 25 April.
πŸ”„ Calendar Converter
Convert any date between the Julian and Gregorian calendars, with its Julian day number and day of the week. Handles BC years (it labels the era on output).

Why is the date of Easter so hard?

The rule sounds simple β€” Easter is the first Sunday after the first full moon on or after the spring equinox β€” but every clause hides centuries of difficulty.

1. Even the first Easter's date is debated

The Gospels disagree on whether the Last Supper was the Passover meal. The Synoptics (Matthew, Mark, Luke) present it as a Passover meal, with Jesus dying on 15 Nisan; John places the crucifixion on 14 Nisan, the Day of Preparation, before the festival. This is the classic problem treated in Joachim Jeremias' The Eucharistic Words of Jesus. It matters here because the choice decides which year's astronomy fits.

2. The Quartodeciman controversy

In the second century the churches of Asia Minor kept Pascha on 14 Nisan whatever the weekday, following the Jewish reckoning, while Rome insisted on the following Sunday. The dispute grew so sharp that Victor of Rome (c. 190) threatened to break communion with the East over it.

3. Nicaea cuts the cord with the Jewish calendar

The Council of Nicaea (325) ruled that Easter should be computed independently of the Jewish calendar β€” a Sunday, after the first full moon on or after the equinox. But it did not hand down a formula, which left the how open.

4. Whose equinox? Whose full moon?

Alexandria fixed the equinox at 21 March and used the 19-year (Metonic) lunar cycle; Rome long used 18 March and a different cycle. For centuries the two great sees could announce different dates for the same Easter. Unifying the calculation took the work of Dionysius Exiguus (525) and, in the West, the Venerable Bede.

5. The calendar beneath it all: Caesar and Sosigenes

All of this reckoning rests on a calendar that was itself a reform. By the first century BC the old Roman calendar β€” a patchwork of months with leap-months inserted by hand β€” had fallen badly out of step with the seasons. Julius Caesar, advised by the Alexandrian astronomer Sosigenes, replaced it with a solar calendar of 365ΒΌ days, taking effect in 45 BC. This Julian calendar governed the Western world for more than sixteen centuries, until Pope Gregory's correction. It is the calendar on which all the historical dates in this Almanac are reckoned.

6. The drift, and the Gregorian reform

The Julian year is about eleven minutes too long, so over a millennium its dates slipped against the seasons. By 1582 the equinox had drifted ten days, and Pope Gregory XIII reformed the calendar β€” dropping ten days and adjusting the leap-year rule. Most of the East kept the Julian reckoning, which is why Eastern and Western Easter still differ today, sometimes by over a month.

The ecclesiastical moon vs. the sky

To keep Easter calculable, the Church uses a tabular ("ecclesiastical") moon and a fixed equinox of 21 March β€” neither of which is the real astronomical event. The Moon Phases tool shows the gap: the ecclesiastical Paschal full moon usually agrees with the astronomical full moon to the day, but not always.

The year of the crucifixion

The two serious candidates are AD 30 (Friday 7 April) and AD 33 (Friday 3 April), each a Friday at Passover under Pontius Pilate. Weighing the chronology of the ministry (John names at least three Passovers, and Luke dates John the Baptist to Tiberius' fifteenth year) together with a lunar eclipse visible from Jerusalem on the evening of 3 April AD 33, this site assumes AD 33. The Passover tool's Passion-era view reproduces that Friday-3-April full moon to the day.

A note on accuracy

Moon positions are computed with Jean Meeus' methods and match the U.S. Naval Observatory to within about two minutes for the modern era. The lunar model holds up remarkably well even in antiquity β€” our 4 BC full moon matches NASA's eclipse catalog to about eight minutes. The dominant uncertainty deep in BC is Ξ”T (the gradual slowing of the Earth's rotation), which can shift the time of day and, occasionally, the calendar date. The date of Easter itself, by contrast, is computed by exact integer arithmetic and is correct for every year.

For the Passover reconstruction, remember that the ancient Hebrew calendar was observational β€” set by sighting the new moon and the ripening of the barley β€” so even a perfect astronomical full moon is not a guarantee of the day Passover was actually kept. These tools reconstruct the sky; they do not replace the historical record.

Sources